The constant demand for increasing power densities of the power converters used today for supplying electronics with power causes the power converter circuits to operate at higher levels of stress. In addition, due to modern integrated electronics a demand for different supply voltages arises. In a modern electronic circuit it is rather common that the circuit needs a supply of e.g. 1.2 V, 1.5 V, 1.8 V and 3.3 V. This diversity of power rails have caused many designers to use intermediate bus power architectures using multiple on-board power converters.
Therefore, the importance of power converters is continuously increasing and the demand for higher efficiency is also continuously increasing.
To convert an input voltage to a different output voltage a voltage converter is needed. The most common type of voltage converters are switched mode voltage converters. A simple switched mode voltage converter comprises an input voltage terminal, an externally controlled switch, an inductor, a capacitor and a diode. The basic principle of such a switched mode voltage converter is that by means of the externally controlled switch the charging and discharging of the capacitor and the inductor is controlled and used for the conversion of the input voltage at the input terminal. If the external switch is efficient in terms of switching time and other losses the switched mode voltage converter becomes very efficient. However, some components of the voltage converter usually exhibit some losses, for example the core of the inductor imposes some limitations on the voltage conversion due to magnetic saturation thereof. Also the externally controlled switch that often comprises a MOSFET transistor imposes some limits on the maximum allowed switching voltage and current.
An example of such a switched mode voltage converter is a “buck” converter for down conversion of the input voltage. Such switched mode voltage converters are efficient and needs a minimum of large passive components compared to the older linear types of voltage regulators.
High reliability of the power converters is of course important; this demand often results in a safety margin of at least 15% for the power converters in nominal use. This means that a power converter only uses 85% of its nominal rating. The safety margin causes the power converters to become unnecessary large and expensive. These safety margins further cause the power converters to operate in a non-optimum way and this causes unnecessary energy losses due voltage conversion inefficiency.
A feasible way to operate power converters in a more optimum way is to utilize adaptive bus voltage. Adaptive bus voltage is implemented by having a first controllable power converter that feeds an intermediate bus voltage to a second power converter used for supplying the load with power. The first power converter adjusts the intermediate bus voltage to match the load of the system. Thereby, allowing the power converters to operate in a more optimum way. However, if the bus voltage is lowered and the demand for power suddenly increases, a temporary power shortage may occur that jeopardizes the functionality of the system. Therefore, the intermediate bus voltage is not allowed to be adjusted to such low levels that the actual load of the power converter suggests. Thus, the power converter is not allowed to operate using the optimum bus voltage.